I have a 24" Magnaflow resonator......zero restriction and performance loss. Just about any length in a 2.5" (or 3" with reducers) that will fit behind the cat before the first intermediate pipe turn will work.
My 24" really taxed the limits. Had to cut the flanges to make it fit. 12-18" should be fine.
Gaping hole........no restriction here.
Fitting it in (with cut flange for reference):
Jerry welding it up. 2000SilverBullet (Paul) in the background:
Corner brace welding:
I'm lucky. I had this done in Corvallis at Jerry'* Precision Muffler. Jerry is an AWESOME guy, gave us his time for our WCBF Tech Day, and gave me a great price break. We actually had 3 cars on the lift that day, and he even fixed driverjohn'* exhaust leak while he was at it.